1. Field of the Invention
This invention relates generally to a system and method for providing an increased power response time in a fuel cell vehicle and, more particularly, to a system and method for providing an increased power response time in a fuel cell vehicle by maintaining a high compressor speed while the vehicle brakes are being applied.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
For certain vehicle operations, it is desirable that the vehicle provide high performance by minimizing the throttle response time, i.e., the time from when the vehicle operator requests power from the fuel cell stack to when the fuel cell stack is able to deliver the power. As is well understood in the art, there is a certain lag between when power is requested from the fuel cell stack in a fuel cell system until when the fuel cell stack is able to deliver the power. For example, the compressor that provides the cathode air to the cathode side of the fuel cell stack is limited in its ability to immediately provide enough air when high power is commanded from the fuel cell stack. A centrifugal air compressor typically used in a fuel cell system has a slower transient characteristic than the hydrogen injector for the anode side. For a 93 kW net power fuel cell stack, it takes about 1.2 seconds to increase the speed of the compressor from idle (20,000 RPM) to full power operation (80,000 RPM).
Not only is there an inherent lag time while the compressor spools up to the desired speed, the power from the fuel cell stack is also selectively distributed between the traction system of the vehicle and the compressor to provide the cathode air.
A driver who is familiar with an internal combustion engine vehicle may push the vehicle accelerator while waiting for a traffic signal or keep pushing the vehicle accelerator pedal while reducing the speed of the vehicle at a corner. However, with the current control strategy for a fuel cell vehicle, the speed set-point of the air compressor will be decreased. Consequently, after the driver releases the vehicle brake pedal, the fuel cell power system cannot deliver high power instantaneously.